In order to meet competition, it is necessary that display devices operate with materials which can be driven by a low voltage and which are very low in cost. Liquid crystals are very promising as display materials for subject devices since they can be driven with low voltages and, especially, in the field-effect type of devices, consume virtually no power, since the current drawn is almost zero. As a result, a number of liquid crystal devices have already been developed and used.
The liquid crystal materials are all organic compounds. While most compounds have two transition points at which they go, respectively, from a solid to a liquid and from a liquid to a vapor, liquid crystal materials have three transition points; in other words, they can exist in four different states. As the temperature is raised, liquid crystals which are initially in the solid state go to the liquid crystal state in which they show the ordering of crystals but flow as readily as true liquids. As the temperature is raised a clearing point is reached. Above this temperature the molecules have enough energy so that the ordered state is completely destroyed and the material now becomes a true liquid. Then, as with ordinary compounds, as the temperature is raised the liquid eventually becomes a vapor.
As to liquid crystal materials in the mesomorphic phase, that is, between the melting point an the clearing point, the liquid crystals fall into three classes, namely smectic, nematic and cholesteric. Liquid crystal materials used in display cells at the present time are all of the nematic type; the nematic liquid crystals can further be subdivided into those which function by the light scattering mode and those which depend on the field effect. The present invention relates to liquid crystal compositions used in display cells depending on the field effect.
A large number of types of nematic liquid crystal materials and compositions have already been discovered. Conventional liquid crystal materials, however, suffer from a number of defects summarized as follows:
1. A common colorless liquid crystal material is that known as the Schiff base. The grouping characteristic of the Schiff base is positioned in the center of the molecule, namely, between two benzene groups which are substituted in the para position. This type of material is very difficult to protect from decomposition because it is subject to attack both by moisture and by oxygen.
2. Liquid crystal materials having an azoxy group between the two benzene rings are relatively stable so far as moisture and oxygen are concerned. However, compounds of the azoxy type absorb light in the visible range. This light degrades them, and moreover, these compounds are colored. Consequently, in order to protect them it is necessary to use a colored filter so that the whole display portion of the cell is colored. The color must be suitable to the overall design of the timepiece or other articles in which the display cell is to be used, and, furthermore, the display contrast is weakened so that it is more difficult to discern the display at low light levels.
3. Liquid crystal compounds having the ester group between the benzene rings (hereinafter referred to as ester liquid crystals) are free from the disadvantages mentioned above. However, in general, the ester liquid crystals known to date are operable over so narrow a temperature range that they cannot be widely used. For example, a liquid crystal composition having the general formula ##STR1## in which R is a linear-chain alkyl group is typical of such compounds. However, when used alone, or when a mixture of compounds having this formula are used, the composition has the deficiency that the clearing point (hereinafter referred to as CP) at which the composition becomes a pure liquid is below 50.degree. C, this temperature limit being unsuitably low for many applications.
Further, compositions consisting of ester liquid crystals having the general formula of ##STR2## in which R and R' are normal alkyl groups, in combination with one or more liquid crystal compounds having the general formula ##STR3## also have been considered. However, the CP of such compositions is also below 50.degree. C so that compositions containing at least one of each type of material described do not meet the requirements. Another disadvantage of this type of composition from which the Schiff base types and the azoxy types do not suffer is that on chilling they do not remain in the super-cooled state for a substantial period of time, so that it is difficult to use such compositions in regions where the temperature can drop below, for instance, 0.degree. C.
4. Another type of liquid crystal material which has been tried in combination with the nitrile described above is that represented by the general formula ##STR4## where R and R' are normal alkyl groups. However, the CP of the compositions was still below 50.degree. C.
5. In a further attempt to raise the CP, ester liquid crystals were tested in combination with the nitriles where the liquid crystals were substituted in the para position of one of the benzene rings with an acyl group, the liquid crystal material having the general formula ##STR5## R and R' again being normal alkyl groups.
Also, compounds of the type ##STR6## were tested in combination with nitriles. Both of these types of compounds raised the CP of the composition above 50.degree. C as desired. However, on a life-test it was found that the ester or carbonate group at the end of the molecule interfered with the orientation of the molecules so that these compounds were unsuitable for use in field-effect display cells.
As is evident, then, it would be desirable to be able to provide liquid crystal composition which operate at a low voltage, are colorless, and are stable against moisture, oxygen and light. In addition, the liquid crystal materials must be nematic and suitable for use in field-effect display cells. Most important, a wider operating temperature range is needed.